1. Field of the Invention
The present invention relates to copper foil for printed wiring boards and to a method of surface treatment suitable for the production of the objective copper foil.
The present invention further relates to a non-cyanide copper-zinc electroplating bath which is suitably used for the surface treatment of copper foil for printed wiring boards.
2. Description of the Related Art
As electronic parts to be mounted on printed wiring boards have been downsized, densified and improved in performance, the requirement for the quality of copper foil to be used as the material of conductor circuits is becoming more strict, requiring highly reliable properties.
The most basic property required is high bonding strength between copper foil and resinous base materials. In addition to ensuring high bonding strength just after the lamination of copper foil and resinous base materials by heating and pressing, maintaining the high bonding strength even after exposure to hard environmental conditions by, for example, dipping in chemicals, such as acids or alkalis, or heating, is the very important problem that should be solved for attaining high quality reliability.
A conventional means for solving the above-described problem is surface roughening of the bonding surface of copper foil to be bonded with resinous base materials, for example, by electrodeposition of copper particles on the surface of cathodic copper foil using a copper plating bath. Such a treatment gives copper foil an increased surface area, which conjointly with the anchoring effect of the copper particles, improves bonding strength remarkably.
However, the surface roughening alone cannot improve other bonding strength characteristics, such as chemical resistance or heat resistance, and the deterioration rate of bonding strength is too high to satisfy practical requirements.
To decrease the deterioration rate of bonding strength, various surface treatment layers are formed on the roughened surface. There have been proposed many surface treatments on the roughened surface of copper foil. For example, there were proposed a copper foil provided on its roughened surface with a chromate coating [Japanese Patent Application Kokoku Koho (publication) No. 61-33908] and a copper foil provided on its roughened surface with a zinc coating and a chromate coating in that order [Japanese Patent Application Kokoku Koho (publication) No. 61-33906]. These conventional copper foils improve some of the above-described properties but, in some cases, provide adverse effects.
Particularly, the copper foil having a chromate layer cannot sufficiently improve the bonding strength after a long-time heating. The copper foil provided with a chromate layer on a zinc coating somewhat improves the bonding strength after heating, but causes a serious deterioration of the bonding strength after dipping in hydrochloric acid that is an essential chemical resistance, and the deterioration rate does not satisfy the practical level necessary for achieving high quality and high reliability.
In addition to the bonding strength characteristics, copper foil for printed wiring boards has stringent requirements for electrical characteristics. Considering that the width and gap of copper foil conductor circuits of printed wiring boards are becoming narrower and finer and resinous insulation layers are becoming thinner increasingly, excellence in migration resistance, which is a necessary electrical characteristic, is an essential condition.
For example, metal-base printed wiring boards the substrates of which are metal bases, such as an aluminum plate or an iron plate, a thin insulation layer, such as a resinous insulation layer, is interposed between the metal-bases and copper foil. In cases where such wiring boards are produced by using copper foil having the above-described conventional surface treatment layers, there occurs due partially to the thin insulation layer the disadvantage that migration makes rapid progress.
Migration is a phenomenon in which circuit copper foil is ionized and liquated in the presence of moisture or water by the potential difference between circuits or circuit layers of printed wiring boards and, as the time goes, the liquated copper ion is reduced to metal or compounds and grow in a dendritic form. When the growing branches reach other metallic material, there occurs a fatal defect in quality, namely a short circuit. That is the "short circuit due to copper ion migration".
To prevent such phenomenon, it is desirable, but almost impossible at present, to use a completely non-hygroscopic material for the polymeric insulating layer or base material that is laminated with copper foil or to make a condition free from the penetration of water.
There are proposed copper foils having copper alloy coating, which exhibit relatively good migration resistance. For example, a copper foil having on its surface a brass coating (copper-zinc alloy coating) is known [Japanese Patent Application Kokoku Koho (publication) No. 51-35711]. Although the copper foil virtually satisfies required properties, such as bonding strength, there is room for improvement. As to the migration resistance, the copper foil seems to be hopeless of improvement in insulation properties because the brass coating is a copper-zinc alloy containing copper as the main ingredient and no measure is taken to prevent the migration of copper ion. Further, the copper foil involves safety and sanitary problems in the characteristics and production of copper foil, such as the high risk of environmental pollution, since the brass coating is electrodeposited by using a plating bath containing cyanides.
In Japanese Patent Application Kokai Koho (laid-open) No. 59-50191 is disclosed another method of plating an alloy of copper and zinc on copper foil by using a plating bath, which however has the shortcoming that the plating bath is so unstable as to form precipitates.